1. Cross-reference to Related Applications
This application is relevant to my prior filed pending applications Japanese Patent Application Nos. Hei 2-138990 and 1-202563 filed with the Japanese Patent Office on May 28, 1990 and Aug. 3, 1989 respectively.
2. Field of the Invention
This invention relates to a readout circuit using a level slice circuit for reading out magnetic records from media such as floppy disks (FDs) and a magnetic recorder using the readout circuit.
3. Description of the Related Arts
FDs are used widely as external storage media for computers, wordprocessors, and the like. The surface of an FD is formed with a track 200 like a concentric circle, as shown in FIG. 4A, when the FD is formatted. The track 200 is divided into a preamble 202, a number of sectors 204, and a postamble 206.
Each of the sectors 204 is further divided into an ID field 208 and a data field 210 between which a gap 212 is provided. The ID field 208 is an area in which ID information is recorded when the FD is formatted. As illustrated in FIGS. 4A and 4B, the ID field 208 contains a Sync (synchronous) field 214, AM (address mark) 216, ID (identification) 218, and CRC (cyclic redundancy check code) 220. The data field 210 is an area which stores data when the FD is actually used for data storage. Thus, a write is made into the ID field 208 and the data field 210 at different points of time; therefore, the amplitude of each signal (reproduction amplitude) obtained during read cycle may become unequal to that during a write cycle because of being off track (which means that a magnetic head is out of place) or various settings.
FIGS. 4Bi and 4Bii are each an example of the ID field 208 on an 8-inch FD; the former is a record example on a single density FD and the latter is a record example on a double density FD. On the single density FD, the ID field consists of a 6-byte Sync field 214, 1-byte AM 216, 4-byte ID 218, and 2-byte CRC 220, namely, the ID field length is 13 bytes. A 6-byte pattern of "00" is recorded in the Sync field 214 and one byte of "FE" in the AM 216. On the double density FD, the ID field consists of a 12-byte Sync field 214, 4-byte AM 216, 4-byte ID 218, and 2-byte CRC 220, namely, the ID field length is 22 bytes. A 12-byte pattern of "00" is recorded in the Sync field 214 and a 3-byte pattern of "Al" and one byte of "FE" in the AM 216.
Reading out information from an FD requires an FD readout circuit containing a magnetic head. Information or data is recorded on an FD by magnetizing the surface of the FD. A magnetic head is used to detect magnetization for signal output. A readout circuit is required to process the signals obtained through the magnetic head for providing necessary information.
FIG. 5 shows a block diagram of a readout circuit according to a first conventional example and FIG. 6 shows the operation of the readout circuit. The circuit shown in FIG. 5 has a magnetic head 12 for reading out information recorded as horizontal magnetization 100 on the surface of an FD 10 (FIG. 6A). The magnetic head 12 is connected to a preamplifier 14 which amplifies signals obtained through the magnetic head 12. The preamplifier 14 is followed by a low-pass filter (LPF) 16 which extracts low-frequency components from the output of the preamplifier 14. A signal 102 obtained through the LPF 16 becomes a signal having a peak when the direction of the horizontal magnetization 100 is reversed (FIG. 6B).
The LPF 16 is followed by a differentiator 18, followed by a comparator 20, followed by a time domain filter 22, followed by a pulse shaper 24. The differentiator differentiates the signal 102 for outputting a signal 104. Therefore, the signal 104 crosses zero when the magnetization 100 is reversed (FIG. 6C). The comparator 20 detects this zero crossing and outputs a square wave signal 106 which makes the high-to-low or low-to-high transition at each zero cross point (FIG. 6D). The pulse shaper 24 takes the signal via the time domain filter 22 and detects a rising or falling edge of the square wave signal 106 for outputting a pulse signal 108 (FIG. 6E).
The time domain filter 22 is a filter adapted to filter the signal 106 in a time domain for removing the effect of saddles. A saddle is a peak caused by approach to zero of the signal 104 provided by the differentiator in an area where magnetization is constant; it causes a waveform like a spike to be generated on the signal 106. The time domain filter 22 is a digital filter adapted to remove the spike-like waveform; it is a circuit generally used in FD readout circuits.
The first conventional example circuit contains a problem of a small saddle margin, namely, it cannot remove the effect of extremely large saddle. To solve such a problem, the applicant has proposed FD readout circuits having a level slice circuit as described in Japanese Patent Application Nos. Hei 2-138990 and 1-202563.
FIGS. 7 and 8 show block diagrams of circuits according to a second conventional example and FIG. 9 shows the operation of the readout circuit. This example readout circuit has a similar configuration to those previously proposed. The configuration of the conventional readout circuit has a feature of a given improvement made to a level slice circuit used in a hard disk readout circuit with a comparatively constant readout amplitude so that the level slice circuit can be applied to an FD readout circuit with a larger fluctuation in the readout amplitude than the hard disk readout circuit.
As shown in FIG. 7, the conventional readout circuit is provided with a level slice circuit 26 and a delay circuit 28 in place of the time domain filter 22 shown in FIG. 5. The level slice circuit 26 includes full-wave rectifying circuits 30 and 32 each of which full-wave-rectifies the LPF16 output, as shown in FIG. 8. The output terminal of the full-wave rectifying circuit 30 is connected via a smoothing capacitor 34 to the positive (+) input terminal of a comparator 36 and the output terminal of the full-wave rectifying circuit 32 is connected to the negative (-) input terminal of the comparator 36. The output terminal of the comparator 36 is connected to the delay circuit 28.
In the conventional example, when a signal 142 (FIG. 9A) is supplied from the LPF 16 to a differentiator 18 and the level slice circuit 26, the full-wave rectifying circuits 30 and 32 of the level slice circuit 26 full-wave-rectify the signal. The full-wave-rectified waveform output by the full-wave rectifying circuit 32 is fed into the comparator 36. On the other hand, the full-wave-rectified waveform output by the full-wave rectifying circuit 30 is smoothed through the smoothing capacitor 34 and then fed into the comparator 36. The comparator 36 compares the full-wave-rectification value from the full-wave rectifying circuit 32 with the smooth value and slices the former level with the latter level as a slice level. A square wave signal, which goes high when the full-wave-rectification value exceeds the slice level and goes low when the value falls below the level, is supplied to the delay circuit 28 which then delays the square wave signal by the delay time which is equal to .DELTA.t of the differentiator 18. The resultant signal (window waveform) 146 is as shown in FIG. 9B.
Signal 144 output by the differentiator 18 takes a waveform (FIG. 9C) as shown in the first conventional example (FIG. 6C). The comparator 20 detects the zero-cross points of the signal 144, only when the signal 146 is high, as zero-cross points related to a change in magnetization 100 (low level mask), and generates a signal 148 not affected by saddles, as shown in FIG. 9D. The pulse shaper 24 detects edges of the signal 148 (FIG. 9E) and outputs read data 152 (FIG. 9F).
Thus, according to the configuration shown in FIG. 7 previously proposed by the applicant, there is provided a readout circuit with a large saddle margin, which is applicable to high-density FDs of 4 MB, etc.
However, the previously proposed configuration is easily affected by a rapid amplitude change although it has a large saddle margin. Since information is recorded in ID and data fields on an FD at different points of time, the reproduction amplitudes may differ because of being off track, etc., as described above. Since full-wave-rectified waveforms are smoothed to determine the slice level in the previously proposed configuration, it is hard to follow up the rapid drop in reproduction amplitude; just after the amplitude drops, so-called data missing is prone to occur in read data. Such missing data is undesirable particularly for appropriate reading of a Sync field.